Camera geometric calibration, namely the conversion of image pixel coordinates to real-world coordinates, is a critical step for various image-based and video-based transportation applications such as speed enforcement, on-street parking, etc. The most straightforward approach is to temporarily lay out visible markers with known real-world dimensional measurements, manually take an image/video of the scene, manually identify the pixel coordinates of those markers, and then use standard camera calibration techniques to derive appropriate transformations. Such a method may be labor intensive and/or disruptive to traffic flow.
Regardless of how the initial camera calibration is done, over time the accuracy of the calibration may deteriorate due to focal shifts, camera alignment or other factors. Eventually it will no longer be able to perform the required function of converting image pixel coordinates to real-world coordinates with sufficient accuracy.